A sofa is composed of a seat mattress and a seat back. To provide the sofa with more functions, there are sofas with the seat mattress and the seat back connecting together by a hinge mechanism, when the seat mattress and the seat back intersect with an angle, the sofa functions as a seat sofa for people to seat and to rest; when the seat back and the seat mattress are unfolded using the hinge mechanism, the sofa functions as a bed for people to sleep.
However, the hinge mechanism to connect the seat back and the seat mattress in some cases is a complex structure with high manufacture cost and inconvenient assembly, in other cases, at least two persons are needed at the same time to operate from two sides of the sofa to rotate the seat back, thus making the sofa bed difficult to operate.